Polycrystalline diamond films whose microstructure typically consist of crystallites with sizes on the order of microns have been synthesized by a variety of chemical vapor deposition (CVD) techniques from methane-hydrogen mixtures. Atomic hydrogen has been recognized to play a crucial role in the growth of phase-pure microcrystalline diamond films by the CVD techniques, typically using hydrocarbons as the carbon source. Atomic hydrogen is thought to play a number of roles including abstraction reactions, termination of carbon dangling bonds, and regasification of nondiamond materials at the growth surface. Reducing the concentration of hydrogen while continually increasing the hydrocarbon content of the plasma normally causes the growth of nondiamond phases and eventually the complete absence of the diamond phase. The grain size, surface morphology, and surface roughness of the polycrystalline diamond films prepared from hydrogen-rich plasmas depend strongly on the film thickness. Generally, the thicker the film, the larger the grain size and the rougher the surface of the film. This behavior is generally ascribed to growth competition between differently oriented grains, with grain growth in turn being strongly correlated with surface roughness as discussed in more detail below. Many applications of CVD diamond films, however, require smooth surfaces, which are not readily prepared from hydrogen-rich plasmas. The ability to control the microstructure and the surface morphology of diamond films, therefore, could be important for tailoring this unique material to a variety of applications.
It has been found that a number of properties, including surface morphology and crystal orientation of microcrystalline diamond films, depend on a variety of factors such as the nucleation process and film deposition conditions. Microcrystalline diamond films grown from randomly oriented nuclei exhibit columnar growth, which is caused by an xe2x80x9cevolutionary selectionxe2x80x9d of crystallites. Because crystals with a direction of fastest growth more or less perpendicular to the substrate grow at the expense of less favorably oriented ones, only a few crystallites survive, and a highly textured film consisting of larger, columnar crystallites is formed after a longer period of growth. The grain size therefore increases with the thickness of the films, and usually the larger the grains the rougher the surface of the films. Therefore, if it were possible to reduce the grain size in a controlled way, smoother surfaces should result.
In order to understand and control surface morphology and crystal orientation, both the nucleation and deposition processes have been investigated extensively. It has been found that diamond films can be grown with a preferred orientation, such as (111) or (110), by precisely controlling the prenucleation treatments and the deposition process parameters. Moreover, it has been recognized that nitrogen and oxygen additions in the plasmas have a strong effect on the growth morphology of diamond films. It is known that to increase the electron density of the plasma and to modify diamond film morphology, argon has been added to plasmas. Argon has also been used in place of hydrogen in a carbon-oxygen-argon system, but oxygen was found to be a critical parameter for the phase purity of the deposited diamond films. Furthermore, in a microwave methane-hydrogen plasma, noble gases were found to have a profound effect on plasma chemistry, including additional ionization and dissociation. Upon adding a noble gas such as Ar, the emission intensity of various species changes and the growth rate of diamond is enhanced. However, the effect of Ar addition to the microwave discharges, and thus the microstructures of the deposited films at concentrations higher than about 30 vol. % heretofore were unknown.
Recently, it has been reported that nanocrystalline diamond films can be grown from an Arxe2x80x94C60 microwave plasma without adding molecular hydrogen to the reactant gas. Fullerenes such as C60 and C70 have been used as the carbon source for nanocrystalline diamond growth. Fragmentation of the fullerenes in the plasma results in strong Swan band emission due to C2 radicals. The C2 dimer appears to be the growth species for nanocrystalline diamond. Furthermore, nanocrystalline diamond films have also been synthesized from Ar/CH4 microwave discharges, without the addition of molecular hydrogen. Atomic force microscopy (AFM) shows that the surface roughness is in the range of 20-50 nm, independent of the film thickness, thus suggesting that grain size remains in the nanometer range. Some of the unique properties of such nanocrystalline films have been characterized, including their tribiological and electron field emission properties.
The present invention relates to a method that allows control of the microstructure of diamond films grown from Ar/H2/CH4 plasmas. Some of the factors leading to a transition from microcrystalline to nanocrystalline diamond have been determined. The as-grown films produced from Ar/H2/CH4 plasmas with different ratios of Ar to H2 have been characterized by scanning electron microscopy (SEM), micro-Raman spectroscopy, transmission electron microscopy (TEM), x-ray diffraction (XRD), and AFM.
An object of the present invention is to provide a method for controlling the crystallite size of plasma-deposited diamond films with a plasma consisting of hydrogen, inert gas and a hydrocarbon.
Yet another object of the invention is to provide a method of controlling the crystallite size of a plasma-deposited diamond film wherein a three-part plasma of hydrogen, hydrocarbon and inert gas contains at least 40% by volume inert gas.
Yet another object of the present invention is to provide a method of depositing a nanocrystalline diamond film from a three part plasma wherein the ratio of inert gas to hydrogen is not less than 4 and preferably not less than about 9.
Another object of the invention is to provide a method of controlling the growth rate of plasma-deposited diamond films from a three part plasma wherein the pressure of the plasma is maintained between about 55 and about 150 Torr during deposition.
The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.